Fatigue damage detection sensor for structural materials and mounting method thereof

ABSTRACT

Focusing on the application of a predetermined relationship between the length of a fatigue damage detection sensor having a width of a specified shape and a required sensitivity or sensing accuracy (crack propagation rate), the present invention provides a fatigue damage detection sensor for structural materials and mounting method thereof, wherein when the length between a pair of fixing portions for fixing to a surface of a structural material M, both ends of a sensor body ( 2 ) sandwiching a notched portion ( 5 ) is defined as 2H, the length of a crack C that can propagate from a tip ( 5 A) of the notched portion ( 5 ) is defined as (a), the number of times that a working load acts on the material is defined as N, and the crack propagation rate is defined as da/dN, the length 2H between said fixing portions is set so as to obtain a required sensitivity with which da/dN is proportional to H 0.5m  (m is a constant determined by a material). According to this fatigue damage detection sensor for structural materials and mounting method thereof, the sensor can provide a high sensing performance to stably and reliably determine the level of fatigue damage caused to a structural material, based on the rate and amount of crack propagation.

TECHNICAL FIELD

The present invention relates to a fatigue damage detection sensor forstructural materials and mounting method thereof, and in particular, toa fatigue damage detection sensor for structural materials that sensesthe history of stress and the level of fatigue damage caused to variousstructural materials or members (base materials) such as steel materialswhich are used to build structures such as bridges, iron towers, andother buildings as well as machine structures such as constructionmachines, to mounting method thereof capable of accurately and reliablymounting the fatigue damage detection sensor so as to provide apredetermined sensitivity.

BACKGROUND ART

Conventional structural materials may be fatigue-damaged under a workingload, and it is essential to periodically inspect these materials tomaintain safety.

These structural materials, however, not only have a large number ofinspection items to be constantly checked but also require expertise andexperience for visual inspections, scaffolding for inspections that isrequired due to a possible danger involved in these operations, a largeamount of time for inspections, and the continuance of inspections overa long time period. Thus, there is a need to accurately and efficientlycarry out determination of the fatigue damage condition of thesematerials and sensing of abnormality.

In such inspections or maintenance and management, sensing of fatiguedamage is divided into three historical stages including a first stagein which no crack has not occurred despite an accumulated fatigue, asecond stage in which damage has occurred as a crack due to the fatigue,and a third stage in which the fatigue has occurred to propagate thecrack.

In each of these stages, for example, the first stage requires the levelof fatigue accumulation to be determined to sense the possibility of theoccurrence of a fatigue crack, the second stage requires a generatedcrack to be sensed, and the third stage requires the current propagationof a crack to be determined to predict future propagation and a point oftime at which the material may be destroyed.

However, the history of stress that may cause a crack was conventionallydetermined by checking the design and measuring stress over a specifiedtime period. This determination, however, has been inaccurate and hasrequired a large amount of time and costs. That is, there has not been apractical sensor for sensing not only the occurrence but also thepropagation of a crack or a method for reliably mounting a detectionsensor with a predetermined sensing accuracy and low costs.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in view of these problems, andits object is to provide a fatigue damage detection sensor forstructural materials and mounting method thereof wherein the fatiguedamage detection sensor can be stably reliably mounted on the materialso as to provide a good sensing performance.

Furthermore, another object of this invention is to provide a fatiguedamage detection sensor for structural materials and mounting methodthereof wherein the sensor can be mounted with a predetermined accuracyso as to sense the occurrence and propagation of a crack appropriately.

Yet another object of this invention is to provide a fatigue damagedetection sensor for structural materials and mounting method thereofwherein the sensor allows a crack to be stably generated and propagatedon behalf of the structural material without being affected by externalenvironments such as the temperature and humidity so as to predict thelevel of fatigue damage caused to the structural material based on therate or amount of propagation.

That is, the present invention focuses on a predetermined relationshipbetween the length of the fatigue damage detection sensor and a requiredsensitivity or sensing accuracy (crack propagation rate) and attempts toincrease the mean stress beforehand by applying this relationship,forming a fatigue pre-crack in the fatigue damage detection sensorbeforehand, leveling or relieving a residual stress, and applying aninitial stress prior to mounting. A first invention is a fatigue damagedetection sensor for structural materials for sensing the stress historyof a structural material that is subjected to a working load as well asthe level of fatigue damage caused to the material, characterized inthat the sensor comprises a detection sensor body configured by arectangular panel-shaped member that can be mounted on a surface of thestructural material and that has a specified width and a predeterminedlength, the detection sensor body including a notched portion on one orboth length-wise sides thereof or inside the plate-shaped member; and acrack detection means located in front of the tip of the notched portionand provided in the sensor body and in that when the length between apair of fixing portions for fixing to the surface of the structuralmaterial, both ends of the sensor body sandwiching the notched portionis defined as 2H, the length of a crack that can propagate from the tipof the notched portion is defined as (a), the number of times that aworking load acts on the material is defined as N, and the crackpropagation rate is defined as da/dN, the length 2H between the fixingportions is set so as to obtain a required sensitivity with which da/dNis proportional to H^(1.5).

The above described notched portion has a fatigue pre-crack formed atits tip, and a residual stress is relieved from the fatigue pre-crack toincrease the mean stress before the sensor body is mounted on thestructural material.

A second invention is a method for mounting a fatigue damage detectionsensor for structural materials for sensing the level of fatigue damagecaused to a structural material that is subjected to an external stress,characterized by comprising the notched portion formation step offorming a notched portion on one or both length-wise sides of a sensorbody configured by a rectangular panel-shaped member that can be mountedon the surface of the structural material and that has a specified widthand a predetermined length or forming the notched portion inside thepanel-shaped member; the fatigue pre-crack formation step of forming afatigue pre-crack in the notched portion, the residual-stress reliefstep of relieving the residual stress resulting from the fatiguepre-crack formation step, and the sensor mounting step of mounting thesensor body on the structural material with the mean stress of thesensor body increased.

In the sensor mounting step, the detection sensor body, which has beenheated, can be mounted on the structural material.

This pre-heating operation causes the detection sensor body to becontracted when cooled to increase its mean stress due to a differencein temperature and thermal expansion between the sensor body and thestructural material.

An appropriate difference in temperature between the detection sensorbody and the structural material caused by pre-heating has been found tobe about 10° C. or higher, preferably about 30° C.

In the sensor mounting step, the sensor body can be linearly heatedwhile being mounted on the structural material.

This linear heating can also increase the mean stress as in thepreheating operation.

Means for mounting the detection sensor body on the structural materialmay be arbitrary and include adhesion, welding, and bolting, but anadhesion means is preferable due to the easiness of this operation.

Although a step is required that mounts the crack detection means on thedetection sensor body in front of the tip of the notched portion, thisstep may be executed before or after machining the notched portion inthe sensor body.

The crack detection means must only be able to detect a crack and mayinclude a crack gauge, an optical crack detection apparatus, or a crackdetection apparatus using ultrasonic waves.

In a fatigue damage detection sensor for structural materials andmounting method thereof according to this invention, the detectionsensor body with the notched portion formed therein and the crackdetection means can be used to detect fatigue damage caused to thestructural material with a predetermined sensitivity or detectionaccuracy, and the sensor can be mounted so as to execute sensing with astable sensitivity.

In particular, based on the proportionality of the crack propagationrate da/dN to H^(1.5), the fatigue damage detection sensor according tothis first invention can adjust the crack propagation rate da/dN tocontrol a required sensitivity. Thus, by setting the lateral length (ina strain (displacement) constraining method for fixing both ends of thesensor body to the surface of the structural material, the lengthbetween the pair of fixing portions) of the sensor body at apredetermined value, the sensor body can be mounted with a desiredsensitivity.

In the method for mounting a fatigue damage detection sensor accordingto this second invention, the fatigue pre-crack is formed in the notchedportion of the sensor body beforehand, a residual stress generatedduring the formation of the fatigue pre-crack is relieved, and the meanstress is increased before mounting the sensor body. These apparatusescan ensure that a crack stably occurs and propagates without beingaffected by external environments such as the temperature and humidity,thereby enabling the history of stress or accumulated fatigue of thestructural material to be sensed with practical construction costs andworkability using the sensor with the notched portion formed therein.

Even if an actual stress caused by a working load is small, theincreased mean stress allows a crack to occur and propagate, and thepropagation rate of a crack linearly varies relative to the stressintensity factor range ΔK proportional to H^(0.5), thereby enablingsensing with a predetermined accuracy.

A desirable mean stress corresponds to a tensile stress of stress ratio0.6 or more. In addition, by allowing the tensile stress to remain,sensing is possible even in a compressed field of the structuralmaterial.

In this way, when a working load acting on a structural material of abridge or the like causes a crack to propagate up to a predeterminedthreshold, this can be visually confirmed or electrically detected usinga crack gauge or a crack detection gauge or apparatus for an alarm orfurther detailed inspections.

That is, when simply affixed to a structural material, the fatiguedamage detection sensor according to this invention can detect thehistory of stress (history of the magnitude of stress and of the numberof stress cycles) acting on the structural material and record theaccumulated history of stress so that these records can be used todetermine the stress or fatigue accumulated during an arbitrary periodin order to predict fatigue damage that may be caused to the structuralmaterial.

Next, a fatigue damage detection sensor 1 for structural materials andmounting method thereof according to an embodiment of this inventionwill be described with reference to FIGS. 1 to 9.

FIG. 1 is a top and front views of the fatigue damage detection sensorcomprising an indicator plate 2 (a sensor body), a crack gauge 3 that isan example of a crack detection means, and a lateral pair of affixedportions 4 (fixing portions). In the figure, an example of the size ofeach part is described in the unit of mm.

The indicator plate 2 constitutes the body of the fatigue damagedetection sensor 1, is shaped like a rectangle having a specified widthand a predetermined length, and has a generally long V-shaped notchedportion 5 on one side.

The crack gauge 3 is placed so as to be opposed to a tip 5A of thenotched portion 5 so that stress is concentrated at the tip 5A to allowa crack to occur in the indicator 2 sufficiently earlier than in a basematerial M when loaded.

The notched portion 5 need not be formed at the center of the indicatorplate 2. It may be formed on both sides of the indicator plate 2 orinside the plate.

The crack gauge 3 preferably determines the length of a crack in theindicator plate 2 more easily when placed perpendicularly to thepropagation direction of a crack propagating from the tip 5A, and may bea strain gauge 6 or a parallel arrangement of electric resistance wires.A fatigue crack is formed at an end of the tip 5A beforehand, asdescribed below in FIGS. 2 and 3.

The lateral pair of affixed portions 4 are used to fixedly mount thefatigue damage detection sensor 1 on the base material M over apredetermined area. The fatigue damage detection sensor 1 and the basematerial M are integrated together at the affixed portions 4, and thelength of part of the fatigue damage detection sensor 1 located betweenthe affixed portions 4 independently of the base material M is definedas 2H.

The material of the indicator plate 2 and an adhesion for the affixedportions 4 affixed to the base material M may be arbitrary but desirablyhave durability and a weather resistance and allow the correlationshipwith stress acting on the base material M and the crack gauge 3 to bestably determined. In general, the adhesion can be thermally set and theindicator plate 2 is formed as thin as possible.

For example, the indicator plate 2 has a thickness of 1 mm or less and awidth varying depending on the length of the sensor. If 2H is set at 100to 200 mm, the width is 10 to 100 mm.

The length of the indicator plate 2 is determined based on a sensitivityrequired of the damage fatigue sensor 1.

As shown in FIG. 1, when the length of a crack C that can propagate fromthe tip 5A of the notched portion 5 is defined as (a), the number oftimes that a working load acts on the material, and the crackpropagation rate is defined as da/dN, da/dN is proportional to H^(1.5).Thus, the length 2H between the affixed portions 4 is set so as toobtain a predetermined sensitivity.

That is, when both ends (affixed portions 4) of the fatigue damagedetection sensor 1 are fixed and a specified strain ε is applied tothese ends and if the modulus of longitudinal elasticity is defined asE, the stress intensity factor K of the tip of the crack C is expressedas follows:

K=E·ε·H ^(0.5)

When the fatigue damage detection sensor 1 is affixed to the basematerial M having a strain variation range ΔK, the stress intensityfactor range ΔK is expressed as follows:

ΔK=E·Δε·H ^(0.5)

The crack propagation rate da/dN is proportional to about the thirdpower of ΔK, so it is proportional to H^(1.5).

FIG. 2 is a flowchart showing a procedure for mounting the fatiguedamage detection sensor 1. First, the notched portion is machined (stepS1).

No fatigue crack occurs in the machined notched portion 5 under such alow stress as generated in the base material M that is an actualstructural material or a long time period is required before a crackoccurs in this portion.

Thus, a fatigue pre-crack 7 (see the enlarged view in FIG. 3) is formedat step S2.

FIG. 4 is a schematic explanatory drawing of a fatigue pre-crackgenerator 8 for forming the fatigue pre-crack. The fatigue pre-crackgenerator 8 comprises a frame 9, a pair of fatigue damage detectionsensor grabbing portions 10, and a fatigue pre-crack monitoring sensor11 wherein the fatigue damage detection sensor grabbing portions 10 arevibrated at predetermined cycles to generate the fatigue pre-crack 7 inthe notched portion 5 of the fatigue damage detection sensor 1.

Since the stress used to generate the fatigue pre-crack 7 is set higherthan a stress normally occurring in the structural material, the crack Cmay not propagate stably due to a residual stress generated at the tipof the fatigue pre-crack 7.

Thus, the fatigue damage detection sensor 1 with the fatigue pre-crack 7formed therein is annealed to relieve the residual stress at step S3(FIG. 2).

FIG. 5 is a schematic explanatory drawing describing this annealingoperation in brief. The fatigue damage detection sensor is accommodatedin a heating furnace 12 and heated therein at a predeterminedtemperature for a predetermined time period. The fatigue damagedetection sensor 1 is then left and the furnace is cooled for annealingto relieve the residual stress generated due to the formation of thefatigue pre-crack 7.

Returning to FIG. 2, the fatigue damage detection sensor 1 with its meanstress increased is mounted on the base material M at step S4.

That is, since a repeated stress that may occur in a normal basematerial M is not so high, the fatigue crack C may not propagate stablyin the fatigue damage detection sensor 1. The fatigue stress C has beenconfirmed to propagate stably even under s low stress if the mean stressof the fatigue damage detection sensor 1 is increased in advance. Themean stress must be at least 30 MPa.

This mean stress can be applied by, for example, applying an initialstress. The fatigue damage detection sensor 1 can be preheated beforemounting on the base material M by means of adhesion, welding, or othermechanical fixing means such as bolting.

Alternatively, after the fatigue damage detection sensor 1 is mounted onthe base material M by adhesion, welding, or bolting, the portion of thesensor corresponding to the indicator plate 2 is linearly heated andthen cooled to allow the indicator plate 2 of the fatigue damagedetection sensor 1 to be contracted in order to apply the initialstress.

FIG. 6 is a schematic explanatory drawing showing this linear heatingoperation. For example, after the fatigue damage detection sensor 1 hasbeen fixed to the base material M by welding fixing portions 13, theportion of the fatigue damage detection sensor 1 corresponding to theindicator plate 2 is heated in the width-wise direction (a linearlyheated portion 14) and then cooled and contracted to generate an initialstress therein in order to increase the mean stress.

In addition, FIG. 7 is a schematic explanatory drawing showing thebolting operation. Bolts 16 are inserted into the indicator plate 2 in alateral pair of bolting areas 15 (fixing portions) to fix the fatiguedamage detection sensor 1 to the base material M. Long holes 17,however, are formed in one of the bolting areas to enable the level ofthe mean stress to be adjusted based on their positions relative to thebolts 16.

FIG. 8 is a graph showing the stress intensity factor range ΔK vs. thecrack propagation rate da/dN in cases where an initial stress is and isnot applied to the indicator plate 2. This graph shows that theapplication of the initial stress increases the mean stress to provide alinear property and allows the crack C to occur and propagate even undera small stress, whereas no crack C occurs under a small stress if theinitial stress is not applied.

In this way, this indicates that a predetermined sensing accuracy andsensitivity (the stress intensity factor range ΔK and the gappropagation rate da/dN) can be obtained by setting the length H in theindicator plate 2 at an arbitrary value.

FIG. 9 is a graph showing a preheating operation for the fatigue damagedetection sensor 1 and temporal changes in the temperature of thesensor, showing how the temperature of each part of the fatigue damagedetection sensor 1 increases.

This preheating method employs, for example, an H-shaped steel as thebase material M, adheres the fatigue damage detection sensor 1 to thecenter of an upper flange 18 of the H-shaped steel, places magnets 19 onthe right and left to the sensor 1 to press the affixed portions 4against the upper flange 18 with a predetermined pressure, places apreheating heater 20 between the magnets 19, and heats the center of theindicator plate 2, as shown in the figure in the graph.

The graph shows that the difference in temperature between the center ofthe fatigue damage detection sensor 1 and the base material M (thecenter of the upper flange 18) becomes almost constant (about 30° C.)after a predetermined time period.

By fixing the fatigue damage detection sensor 1 to the base material Mwhile maintaining this constant temperature difference, the fatiguedamage detection sensor 1 allows the crack C to be stably generated andpropagated and detects it despite a difference from the temperature ofthe external environment (for example, −20° C. to +50° C.).Consequently, the sensor 1 can sense the level of fatigue damage causedto the base material M.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top and front views of a fatigue damage detection sensor 1for structural materials according to an embodiment of the presentinvention;

FIG. 2 is a flowchart showing a procedure for mounting the fatiguedamage detection sensor 1;

FIG. 3 is an enlarged view showing a fatigue pre-crack 7 portion;

FIG. 4 is a schematic explanatory drawing of a fatigue pre-crackgenerator 8 for forming the fatigue pre-crack 7;

FIG. 5 is a schematic explanatory drawing describing an annealingoperation in brief;

FIG. 6 is a schematic explanatory drawing showing a linear heatingoperation;

FIG. 7 is a schematic explanatory drawing showing a bolting operation;

FIG. 8 is a graph showing the stress intensity factor range ΔK vs. thecrack propagation rate da/dN in cases where an initial stress is and isnot applied to the indicator plate 2 configured by a steel material;

FIG. 9 is a graph showing a preheating operation for the fatigue damagedetection sensor 1 and temporal changes in the temperature of thesensor;

FIG. 10 is a top view of the fatigue damage detection sensor accordingto the embodiment;

FIG. 11 is a top view of a specimen to which the fatigue damagedetection sensor according to the embodiment has been affixed;

FIG. 12 is a graph showing the results of fatigue tests;

FIG. 13 is a graph showing the results of the fatigue tests;

FIG. 14 is a graph showing the results of the fatigue tests;

FIG. 15 is a graph showing the results of the fatigue tests;

FIG. 16 is a graph showing the results of the fatigue tests;

FIG. 17 is a schematic explanatory drawing showing a plate that has beensubjected to a specified displacement at both ends to generate a crackon one side;

FIG. 18 is a graph showing the results of the fatigue tests;

FIG. 19 is a graph showing the results of the fatigue tests;

FIG. 20 is a graph showing the results of the fatigue tests;

FIG. 21 is a schematic explanatory drawing describing in detail a methodfor affixing the fatigue damage detection sensor to a structuralmaterial;

FIG. 22 shows the results of fatigue crack propagation tests; and

FIG. 23 is a graph showing the level of fatigue damage and the JSSCfatigue design curve.

BEST MODE FOR CARRYING OUT THE INVENTION

The fatigue damage detection sensor for structural materials andmounting method thereof according to the present invention will bedescribed in conjunction with several embodiments. The presentinvention, however, is not limited to these embodiments.

Fatigue Tests

The fatigue damage detection sensor for structural material according tothis invention was subjected to fatigue tests.

The fatigue damage detection sensors each comprised by SUS304 and anAl—Mg alloy and had a width of 50 mm, a length of 120, 170, and 270 mm,a thickness of 0.5 and 0.3 mm, and a gauge length of 50, 100, and 200 mm(FIG. 10). The standard thickness of the sensors was 0.5 mm and thesensor of 0.3 mm thickness was used to determine the effect of thethickness.

The length of a crack was measured using a crack gauge comprising a 1-mmpitched grid and stuck to the sensor opposite to a tip of a notchportion located on its inner side.

A fatigue specimen of 20 mm thickness was used for the fatigue tests(FIG. 11). The sensor was affixed to the specimen, and load controlledfatigue tests were conducted using a servo-hydraulic testing machine.

Embodiment 1

Both ends (the affixed portions in FIG. 10) of the mechanical notchedsensor, that is, the above sensor were stuck to the fatigue specimenusing an epoxy type adhesive, and this sensor was used to conduct theabove fatigue tests under the conditions of stress range 150 MHa andstress ratio 0.1. FIG. 12 shows the results.

The results indicate that the length of a crack in the sensor wasproportional to the number of stress cycles. They also indicate that thecrack propagation rate does not depend on the crack length.

In addition, a comparison between the sensors of gauge length 100 mm(S100-0.5) and 50 mm (S50-0.5) indicated a significant effect of thegauge length.

The aluminum alloy (A100-0.5) appears to have a slightly higher crackpropagation rate than the SUS304 (S100-0.5). Due to the softness of thealuminum alloy, however, the sensor may be damaged during handling dueto an unexpected deformation. Accordingly, the SUS304 was largely usedfor this embodiment.

In addition, a comparison between the results on the thicknesses of 0.5and 0.3 mm exhibited no effect of the thickness.

Next, FIG. 13 shows the results of the above fatigue tests conductedunder the conditions of stress range 50 MPa and stress ratio 0.1.

A crack occurred in the sensor of gauge length 200 mm, whereas nofatigue crack occurred at the tip of the mechanical notch in the sensorof gauge length 100 mm.

During the tests, when the stress ratio was changed from 0.1 to 0.33,the crack propagation rate significantly increased.

Since a stress generated in a steel structure such as a railway bridgeis 50 MPa or less, a crack must be able to propagate stably even undersuch a low stress as about 20 or 30 MPa. A crack is expected to occurmore easily from the mechanical notch tip by forming a fatigue pre-crackin the sensor and annealing the sensor to relieve the stress.

In addition, the dependency on the stress ratio observed in the fatiguetests on the sensor of gauge length 200 mm is assumed to result from acrack closure behavior. Thus, a stable crack propagation behavior isexpected to be obtained by increasing the mean stress beforehand.

Embodiment 2

A fatigue pre-crack was formed in the fatigue damage detection sensorusing the above method, and the sensor was annealed to relieve thestress. When affixed to the fatigue specimen, the sensor was heated toapply an initial stress thereto. When the sensor is heated, the meanstress increases after cooling due to the difference in temperature andlinear expansion between the sensor of SUS304 and the fatigue specimenof steel.

FIGS. 14 to 16 show the results of the above fatigue tests using thesensors.

The independency of the crack propagation rate on the crack length wasconfirmed again.

A comparison between FIGS. 13 and 15 for the gauge length of 200 mmindicates that the crack propagation rate was significantly accelerateddue to the mean stress. The crack also propagated in the sensor of 100mm gauge length. Besides, the crack propagated even in such a low stressrange as 30 MPa and the incubation period before the crack occurred wasnegligibly short.

Prior to the tests, a strain gauge was stuck to the detection sensor anda released stress was measured after cracking. The measured value was−322 με. That is, 66-MPa stress is assumed to be have been applied.

Embodiment 3

The fatigue crack was found to propagate even under a low stress rangewhen an initial stress (pre-stress) was applied to the fatigue damagedetection sensor to increase the mean stress. Thus, in order toquantitatively determine the value of a required pre-stress, the effectsof the mean stress on the crack propagation properties wereinvestigated.

The test was carried out under the condition that the detection sensorwas stuck to the fatigue specimen without applying any pre-stress and astress ratio was controlled by a fatigue tester. FIG. 18 shows theresults.

The stress intensity factor range was calculated using the followingEquation (1) given for the plate having a single-sided crack (a) causedby a specified displacement Δε (a strain variation range) applied toboth ends of GL (gauge length), as shown in FIG. 17.

[Equation 1] $\begin{matrix}\begin{matrix}{{\Delta \quad K} = {{E \cdot \Delta}\quad ɛ\sqrt{{GL}/2}}} \\{= {{\Delta\sigma}\sqrt{{GL}/2}}}\end{matrix} & (1)\end{matrix}$

(in the equation, ΔK denotes the stress intensity factor range, E isdenotes the modulus of longitudinal elasticity, Δε denotes the strainvariation range, GL denotes the gauge length, and Δσ denotes stress).

There was a tendency that in an area where the stress intensity factorrange is low, the effect of the stress ratio was significant and thatthe crack propagation rate decreases with decreasing stress ratio. Insome of the tests, the crack did not propagate. With a stress ratio of0.67 or more, the crack propagation rate is not affected by the stressratio and remains stable.

This crack propagation rate was defined as a stable crack propagationrate, and the stress ratios were classified into those with a lowercrack propagation rate and those with no crack propagation. FIGS. 19 and20 show these results for sensors having a gauge length of 100 mm and200 mm, respectively. The crack propagation was delayed or stopped whenthe minimum stress and stress amplitude were low.

An area where the crack propagated stably is shown by a solid line.

Assuming that a stress variation range to be evaluated for an actualbridge is 20 MPa or higher, it has been found that the initial stress(pre-stress) required to allow a fatigue crack in the sensor topropagate stably within this range is 70 MPa for the sensor of gaugelength 100 mm and 40 MPa or more for the sensor of gauge length 200 mm.

Embodiment 4

When affixed to a structure, the fatigue damage detection sensor can beheated to introduce a pre-strain based on the difference in temperatureand thermal expansion rate between the sensor and the structure. Thus,in order to provide a constantly stable pre-strain, an attempt was madeto standardize the sensor affixing operation.

As shown in FIG. 21, based on the method of placing a heater on thesensor and holding both ends of the heater using magnetic fixtures,conditions were determined that were required to stably introduce a 600-to 800 με pre-strain.

The crack propagation test was carried out under the condition that thepre-stress was applied to the fatigue damage detection sensor by theabove described method.

As described above, the crack propagation rate has been confirmed to beconstant irrespective of the crack length, so in this test, the stresscondition was changed after a crack propagated about 5 mm. A pulsatingtension having a stress ratio of 0.1 was used as a working load, and thestress range was set between 20 and 70 MPa. Due to the application of ahigh tensile pre-stress, the substantial stress acting on the sensor wasa tension despite the use of a compression stress as a working load.Accordingly, since a crack was expected to propagate despite the use ofthe pulsating compression stress as a working load, a compression testusing a stress ratio of 1.1 was also carried out. FIG. 22 shows theresults of these tests together with the results for the stress ratio of0.67 or more shown in FIG. 18.

The results obtained under the condition of stress ratio 0.67 or morecoincide well with the results for the stress ratio of 1.1, indicating astable crack propagation property. FIG. 22 uses a dashed line to showthe crack propagation property recommended by JSSC, and the results ofthese tests also coincide well with this crack propagation property. Inaddition, to estimate the crack propagation life from given mechanicalconditions, the safest design curve is commonly used that is the upperlimit of the data.

However, to estimate stress history from the amount of crackpropagation, the use of the lower limit provides a safer evaluation.Thus, this embodiment has used a mean design curve expressed by thefollowing equation.

[Equation 2] $\begin{matrix}{{{{a}/{N}} = {1.5 \times 10^{- 11}\quad \Delta \quad K^{2.75}}}{{a}/{{N:{m\text{/}{cycle}}}}}{\Delta \quad {K:{{Mpa}\left. \sqrt{}m \right.}}}} & (2)\end{matrix}$

(in the equation, ΔK denotes the stress intensity factor range)

Embodiment 5

The history of stress acting on a member is detected from the results ofmonitoring performed by the fatigue damage detection sensor according tothis invention, while the level of fatigue damage is directly evaluated.

Next, a method for directly evaluating the level of fatigue damage usingthe fatigue damage detection sensor according to this invention will bedescribed below.

When the crack propagation amount during a monitoring period is definedas Δa and the working load is an equivalent stress Δσ_(eq), the stressintensity factor range for the sensor is expressed by the followingExpression (3) with a difference in modulus of longitudinal elasticitytaken into consideration.

[Equation 3]

ΔK=(197/206)·Δσ_(eq)GL/2  (3)

(in this equation, ΔK designates the stress intensity factor range,Δσ_(eq) designates an equivalent stress, GL designates the gauge lengthin FIG. 10.)

From Equations (2) and (3), the following equation is derived.

[Equation 4] $\begin{matrix}{{\Delta \quad {\sigma_{eq}^{2.75} \cdot \Delta}\quad N} = \frac{\Delta \quad a}{1.5 \times 10^{- 11}\quad \left\{ {\left( {197/206} \right)\quad \sqrt{{GL}/2}} \right\}^{2.75}}} & (4)\end{matrix}$

(in this equation, Δσ_(eq) indicates an equivalent stress, ΔN indicatesthe number of stress cycles, Δa indicates the amount of crackpropagation, and GL indicates the gauge length in FIG. 10.)

That is, the fatigue damage ΔK·(Δσ_(eq))^(2.75) can be directly sensedfrom the crack propagation amount of the fatigue damage detection sensoraccording to this invention.

FIG. 23 shows a comparison between the level of fatigue damagedetermined by the sensor of 200 mm gauge length (H=100 mm) and the JSSCfatigue design curves wherein the crack propagation amount Δa is 1 or 10mm. This figure indicates that the sensor can detect fatigue damagesufficiently early compared to the design curve F and can also be usedto predict the fatigue life.

Since this sensor can detect the damage level in the form of the leftside of Equation (4), the equivalent stress can be easily calculated bydetermining the number of stress cycles during the monitoring period.

Industrial Applicability

As described above, the present invention can obtain an arbitrarysensitivity by setting the length between the fixing portions of thedetection sensor body to adjust the fatigue propagation rate and canmount the sensor on the base material (a structural material of a bridgeor the like) with a predetermined stability and reliability so as toprovide a predetermined sensing accuracy, using various steps includingthe formation of a fatigue pre-crack, the relief of a residual stress,and the mounting of the sensor with its means stress increased.

That is, the fatigue crack propagation property of the fatigue damagedetection sensor according to this invention at a specified strainamplitude has been understood to establish the method for introducing afatigue pre-crack into the sensor and annealing it for stress relief tointroduce a pre-strain into it. Thus, the fatigue damage detectionsensor that can operate stably under a low stress can be obtained todirectly evaluate the level of fatigue damage based on the results ofmonitoring performed by the sensor.

What is claimed is:
 1. A fatigue damage detection sensor for sensing thelevel of fatigue damage caused to a structural material that issubjected to a working load, the sensor comprising: a rectangularlyconfigured, panel-shaped sensor body having a specified width and apre-determined length, the sensor body including at least one notchedportion which is disposed therein and defines a tip; a pair of fixingportions for mounting the sensor body to the structural material, thesensor body having a length 2H between the fixing portions when mountedto the structural material thereby; and a means for crack detectiondisposed on the sensor body adjacent the tip of the notched portion; thelength 2H being set so as to cause a crack propagation rate d(a)/dN tobe proportional to H^(0.5m), (a) being the length of a crack thatpropagates from the tip of the notched portion, N being the number oftimes that the working load acts on the structural material, H beingone-half the length 2H, and m being a constant determined by thestructural material.
 2. The fatigue damage detection sensor of claim 1wherein the sensor body includes a fatigue pre-crack formed therein atthe tip of the notched portion.
 3. The fatigue damage detection sensorof claim 1 wherein each of the fixing portions is fabricated from anadhesive.
 4. A method of mounting a fatigue damage detection sensor to astructural material for sensing the level of fatigue damage caused tothe structural material when the structural material is subjected to anexternal stress, the method comprising the steps of: (a) providing arectangularly configured, panel-shaped sensor body having a specifiedwidth and a predetermined length; (b) forming a notched portion whichdefines a tip in the sensor body; (c) forming a fatigue pre-crack in thesensor body at the tip of the notched portion; (d) relieving a residualstress from the sensor body resulting from the formation of the fatiguepre-crack in step (c), the relief of the residual stress increasing amean stress of the sensor body; and (e) mounting the sensor body to thestructural material.
 5. The method of claim 4 wherein step (e) furthercomprises the step of heating the sensor body prior to the mountingthereof on the structural material.
 6. The method of claim 4 whereinstep (e) comprises linearly heating the sensor body prior to themounting thereof on the structural material.
 7. The method of claim 4wherein step (e) is accomplished through the use of an adhesive.
 8. Amethod for mounting a fatigue damage detection sensor to a structuralmaterial for sensing the level of fatigue damage caused to thestructural material when the structural material is subjected to anexternal stress, the method comprising the steps of: (a) providing arectangularly configured, panel-shaped sensor body having a specifiedwidth and a predetermined length; (b) forming at least one notchedportion which defines a tip in the sensor body; (c) positioning a meansfor crack detection on the sensor body adjacent the tip of the notchedportion; and (d) mounting the sensor body to the structural material viaa spaced pair of fixing portions such that the sensor body has a length2H between the fixing portions, with the length 2H being set so as tocause a crack propagation rate d(a)/dN to be proportional to H^(0.5m),(a) being the length of a crack that propagates from the tip of thenotched portion, N being the number of times that the external stressacts on the structural material, H being one-half the length 2H, and mbeing a constant determined by the structural material.
 9. The method ofclaim 8 wherein step (d) comprises heating the sensor body prior to themounting thereof on the structural material.
 10. The method of claim 8wherein step (d) comprises linearly heating the sensor body prior to themounting thereof on the structural material.
 11. The method of claim 8wherein step (d) further comprises forming a fatigue pre-crack in thesensor body at the tip of the notched portion to relieve a residualstress in the sensor body and increase a mean stress therein.